1. Field of the Invention
The present invention relates to a radiation image storge panel employed in a radiation image recording and reproducing method utilizing a stimulable phosphor.
2. Description of the Prior Art
As a method replacing a conventional radiography, a radiation image recording and reproducing method utilizing a stimulable phosphor as described, for instance, in U.S. Pat. No. 4,239,968, has been recently paid much attention. In this method, a radiation image storage panel comprising a stimulable phosphor (i.e., stimulable phosphor sheet) is employed, and the method involves the steps of causing the stimulable phosphor of the panel to absorb radiation energy having passed through an object or having radiated from an object; sequentially exciting the stimulable phosphor with an electromagnetic wave such as visible light or infrared rays (hereinafter referred to as "stimulating rays") to release the radiation energy stored in the phosphor as light emission (stimulated emission); photoelectrically detecting the emitted light to obtain electric signals; and reproducing the radiation image of the object as a visible image from the electric signals.
In the radiation image recording and reproducing method, a radiation image is obtainable with a sufficient amount of information by applying a radiation to an object at considerably smaller dose, as compared with the conventional radiography. Accordingly, this method is of great value especially when the method is used for medical diagnosis.
The radiation image storage panel employed in the above-described radiation image recording and reproducing method basically comprise a support and a stimulable phosphor layer provided thereon. Further, a transparent film is generally provided on the free surface of the phosphor layer (a surface not facing the support) to keep the phosphor layer from chemical deterioration and physical shock.
The phosphor layer generally comprises a binder and stimulable phosphor particles dispersed therein. The stimulable phosphor emits light (gives stimulated emission) when excited with an electromagnetic wave (stimulating rays) such as visible light or infrared rays after having been exposed to a radiation such as X-rays. Accordingly, the radiation having passed through an object or radiated from an object is absorbed by the phosphor layer of the panel in proportion to the applied radiation dose, and a radiation image of the object is produced in the panel in the form of a radiation energy-stored image. The radiation energy-stored image can be released as stimulated emission by sequentially irradiating (scanning) the panel with stimulating rays. The stimulated emission is then photoelectrically detected to give electric signals, so as to reproduce a visible image from the electric signals.
The radiation image recording and reproducing method is very advantageous for obtaining a visible image as described above, and the radiation image storage panel used in the method is desired to have high sensitivity and provide an image of high quality (high sharpness, high graininess, etc.), as well as a radiographic intensifying screen used in the conventional radiography.
In performing the radiation image recording and reproducing method, the radiation image storage panel is repeatedly used in a cyclic procedure comprising the steps of: exposing the panel to a radiation (recording radiation image thereon), irradiating the panel with stimulating rays (reading out the recorded radiation image therefrom) and irradiating the panel with a light for erasure (erasing the remaining radiaton image therefrom). The panel is transferred from a step to the subsequent step in a transfer system in such a manner that the panel is sandwiched between transferring members (e.g., rolls and endless belt) of the system, and piled on other panel to be stored after one cycle is completed.
The repeated use of the panel comprising transferring and piling causes physical contacts such as a friction between the surface of the panel (surface of the phosphor layer or surface of the protective film) and a surface of other panel (surface of the support), friction between an edge of the panel and a surface of other panel, and a friction between the panel and transferring members (e.g., roll and belt).
As a support material of the radiation image storage panel, desirably employed are plastic films such as a polyethylene terephthalate film and various papers from the viewpoint of flexibility required in the transferring procedure of the panel.
However, the panel having the support made of a polymer material or a paper is apt to be electrostatically charged on its surface owing to the physical contact in the transferring procedure. In detail, the surface (front surface) of the panel is apt to be negatively charged and other surface (back surface) thereof is apt to be positively charged. This static electrification causes various problems in the practical operation of the radiation image recording and reproducing method.
For example, when the surface of the panel is electrostatically charged, the surface of the panel easily adheres to a back surface of other panel and thus adhered panels hardly separate from each other in the vertical direction against the panel surface. Accordingly, those panels are transferred together in layers from the piling position into the transfer system, whereby the subsequent procedure cannot be normally conducted. The read-out procedure of the panel is generally carried out by irradiating the panel with stimulating rays from the phosphor layer-side surface of the panel, and in this procedure, the charged surface of the panel is likely to be deposited with dust in air, so that the stimulating rays are also scattered on the dust deposited thereon and the quality of the resulting image lowers. Moreover, the panel decreases in the sensitivity or the resulting image provided by the panel suffers noise such as static mark when discharge takes place, and a shock is sometimes given to the operator because of the discharge from the panel.
For the purpose of improving the sensitivity of the storage panel, Japanese Patent Provisional Publication No. 56(1981)-12600 discloses that a light-reflecting layer containing a white pigment (e.g., titanium white, basic lead carbonate, zinc sulfide, alumina and magnesium oxide) between the support and the stimulable phosphor layer. For the same purpose for enhancing the sensitivity, there has been proposed that a light-reflecting material such as titanium dioxide, aluminum oxide, silicon oxide and zinc oxide is incorporated into the support made of a plastic film, as described in Japanese Patent Provisional Publication No. 59(1984)-72437. Otherwise, a support of a plastic film is incorporated with a light-absorbing material such as carbon black for improving the quality of an image provided by the panel. However, the amount of carbon black to be incorporated into the support for that purpose is very small, so that even in the case of using the support containing carbon black, the resulting panel is not sufficiently prevented from static electrification on the surface. For example, a commercially available panel having a support containing carbon black (trade name: Fuji CR Imaging Plate, available from Fuji Photo Film Co., Ltd.) has a resistivity of hither than 10.sup.15 ohm on the surface of the support.
With respect to improvements of the above-mentioned static electrification of the panel, there are patent applications for a radiation image storage panel provided with an antistatic layer made of a conductive material and having a low specific surface resistivity (not higher than 10.sup.11 ohm) on the surface of the support not facing the phosphor layer (Japanese Patent Application No. 60(1985)-228418 and a radiation image storage panel provided with an antistatic layer made of at least one conductive material selected from the group consisting of a metal oxide, carbon black and a conductive organic material and having a low specific surface resistivity (not higher than 10.sup.12 ohm) between the support and the phosphor layer (Japanese Patent Application No. 61(1986)-242795).